Bug 1006569 - [privacy] Add checkbox at sign-up

Functionally works fine for me and all the tests pass too.
I will add the unit tests and modify helper text later today and need to discuss about the deletion of user email from database usecase.

Ah, I see, it's a recursion. That's fine but it's still not acceptable to present then with a username with digits appended if they could have had one without. Can you code it so that it first tries a username without any digits and only starts recursing if that won't work?